The rotorblades of modern helicopters are made of composite materials and suffer from wear due to sand and rain erosion as well as overheating by absorbing the sun's infrared rays. Wear degrades the adhesive used in these laminated composites and results in debonding and delaminating the blade's composite skin. Specifically, the high temperatures resulting from the blade absorbing the sun's infrared rays cause bonding deterioration and delamination of helicopter rotor blade components. Debonding is the disintegration of the epoxy or other adhesive materials between spar connections, and delamination is the peeling of layers of the composite skin forming the outer surface of the rotor blade. In addition to the ultraviolet effects of the sun, erosion, poor repairs and repeated high cyclical loading exasperates the problems of wear causing minute openings in the rotorblade's skin. A protector for protecting aircraft, particularly helicopter rotor blades, from absorbing the sun's infrared rays and the accompanied heat build-up in order to keep adhesive material between spar connections from debonding and to avoid delamination from the rotor blade's composite skin is described in U.S. Pat. No. 6,835,045 to Barbee et al. This rotor blade protector includes a cover configured to encircle the length of the rotor blade and a guide form for installing and removing the cover. The inner surface of the cover is coated, by vacuum deposition technology, with aluminum, titanium, or other metals or alloys. A stripper rod is attached to the guide so that aircraft personnel can install and remove the protector from the helicopter rotor blade while standing on the ground. This invention, while helping to protect the rotor blade against overheating, does not help in preventing deterioration due to sand and rain erosion.
A pre-shaped protective layer consisting of an abrasive strip having a mesh bonded to its inside surface to ease its attachment to the surface of the rotor blade is proposed in U.S. Pat. No. 5,174,024 by Sterrett. The abrasive strip is used as a sacrificial layer which is to be replaced as it wears away with usage. While this invention improves the bonding between the abrasion strip and the blade, it does not help to substantially improve the erosion resistance of the strip which will result in the necessity of replacing the strip too often especially when helicopters operate in harsh environments.
Hard wearing surfaces are in common use in various industries, and such hard wearing surfaces are frequently obtained by coating the surface of a tool made of steel or similar metal, or other hard, enduring material, with a layer of hard wearing ceramic substance, such as carbides, nitrides and carbonitrides, or providing a hard microcrystalline diamond coating. There are known methods for obtaining hard wearing coatings, such as for example, having a coating of diamond particles in combination with a carbide or nitride layer and then filling the gaps between the abrasive particles with a softer intermetallic compound. Another known method is vapor deposition of hard-wearing ceramic materials from plasma or by utilizing molten ceramic substances.
A device for yielding hard ceramic surfaces by cathodic arc plasma deposition is described in U.S. Pat. No. 4,851,095, issued to M. A. Scobey et al. on Jul. 25, 1989. The apparatus of Scobey et al. utilizes a high intensity ion flux. Vapor deposition of a hard ceramic material, such as titanium or zirconium nitride on a stainless steel or titanium surface by utilizing a molten evaporant and a hollow cathode, is described in U.S. Pat. No. 5,152,774, issued to W. A. Schroeder on Oct. 6, 1992. The vapor deposition of Schroeder is conducted at relatively low temperature, thus the substrate will have lost little of its initial high strength properties, however, the requirement of low surface roughness of the deposited layer is not addressed by U.S. Pat. No. 5,152,774. In U.S. Pat. No. 4,981,756, issued to H. S. Rhandhawa on Jan. 1, 1991, a method is taught to coat surgical tools and instruments by cathodic arc plasma deposition. The ceramic coating obtained by this technology is a nitride, carbide or carbonitride of zirconium or hafnium, in a single layer of 3-10 μm thickness. U.S. Pat. No. 4,981,756 also refers to various publications describing known equipment for obtaining hard-wearing surfaces by cathodic arc plasma deposition. U.S. Pat. Nos. 5,940,975 and 5,992,268 issued to T. G. Decker et al. on Aug. 24, 1999 and Nov. 30, 1999, respectively, teach hard, amorphous diamond coatings obtained in a single layer on thin metallic blades or similar metallic strips utilizing filtered cathodic arc plasma generated by vaporizing graphite. It is noted that coating thickness in these processes is limited to below 20 μm. Such coatings are used for a wide range of applications: surface engineered medical instruments, cutting and forming tools, protective-decorative, to name a few. Unfortunately these coatings are too thin for application as a protective erosion and corrosion resistive coatings for protective shields of the helicopter rotor blades. In addition these coatings do not have high enough reflectivity to reduce the heat ingested from sun's radiation.
The grain size of deposits obtained in conventional physical vapor deposition (PVD) methods such as cathodic plasma arc, magnetron sputtering or electron beam PVD (EB-PVD) as well as CVD methods may range between 0.5 to 10 μm. Any post-deposition heat treatment which may be required to maintain corrosion resistance of the substrate, may lead to internal stresses in the coating due to differences in the grain size, and can eventually lead to abrasion, spalling, crack formation, grain separation, surface fractures, uneven edges and rough surfaces, and the like, which can drastically reduce the wear resistance and durability of coated objects. None of the above discussed methods are concerned with even grain size and surface structure, and low micro-roughness of the vapor deposited hard, ceramic coatings. Another disadvantage of the above mentioned conventional PVD and CVD technologies is that they are producing the hard, but brittle coatings which have very limited ductility, unable for bending without developing a large cracks, fracturing and delaminations. This makes these coatings non-applicable for such applications as protective shields for the helicopter rotor blades made of thin metal sheets or foils.
Erosion protection coatings for turbomachinery components such as turbine engine compressor blades, vanes, rotor blades and turbine discs are deposited mostly by thermal spray, CVD and PVD methods including cathodic arc evaporation, electron beam evaporation and magnetron sputtering. Thermal spray methods are producing large surface roughness as well as voids and porosity in coating layers detrimental to erosion and corrosion protection, while PVD and CVD methods are known to produce denser coatings having smoother surfaces. Specifically, the TiN erosion resistant coatings and MCrAlY environmental and bond coatings deposited by conventional direct cathodic arc deposition process (without filtration of macroparticles) are reported, for instance, in US Patent Application No. 2004/0126492 by Weaver et al., and described in “Ti—N multilayer systems for compressor airfoil sand erosion protection,” A. Feuerstein, et al., Surface & Coatings Technology 204 (2009) 1092-1096. Cathodic arc deposition process produces large flux of metal vapor plasma featuring metal ions with energies exceeding 40 eV for most of the metal targets, but as a drawback it also generates large amount of microparticles having diameters ranging from nanoparticles to more than 10 μm. As a result, cathodic arc coatings incorporate a large number of macroparticles emitted by the arc cathode target creating a rough surface morphology comprising a large density of isolated bumps and other surface defects which protrude above the surface at 10 to 20 μm and more. The same is true for commercial ultra-thick magnetron sputtering coatings described, for instance, in “Deposition of thick nitrides and carbonitrides for sand erosion protection,” R. Wei et al., Surface & Coatings Technology 201 (2006) 4453-4459, consisting of a noddle-like defects protruding across the coating as a result of the deposition of metal sputtering atoms with very low kinetic energy, typically less than 5 eV and extremely low ionization of metal sputtering flow typically less than 0.1%. Sputtering metal atoms carrying low energy, not exceeding the inter-atomic bonding energy of the substrate atomic lattice, form a network of surface adatoms not having enough energy for surface diffusion, which can adjust the surface energy and smooth unevenness of the growing film. CVD and E-beam evaporating coatings feature large columnar structure resulting in a surface roughness comparable to the width of the columns of about 100 um. These processes require high substrate temperatures, typically above 900 deg C., resulting in a dramatic increase of surface roughness by thermal surface grooving.
In U.S. Pat. No. 6,617,057 issued to Gorokhovsky, a multilayer cermet coating is described which employs alternating metal and ceramic layer. This coating architecture provides high hardness and at the same time secures a necessary elasticity and ductility so the brittle hard ceramic layer will not fail due to bending and deformation of the substrates while a tool is in operation. Using multilayer coating architecture for erosion resistant coatings used for turbomachinery components has been described in U.S. Pat. No. 5,656,364 to Rickerby et al., which is incorporated herein by reference. Using the advanced filtered cathodic arc technology to create the multilayer coating eliminates the problems of surface roughness, produces coatings with extremely low density of growing defects, voids and porosity. This coating was successfully applied to the metal foils for their primary usage as erosion and corrosion protective-decorative coating for exterior architectural parts which is described in “Vacuum Cermet Coatings on Coiled Materials,” V. Gorokhovsky, Proceedings of the Fourth International Conference on Vacuum Web Coating, (ed. by R. Bakish), Reno, Nev., 1990. The disadvantage of this type of coating is its limited thickness range, which makes it non-applicable as erosion and corrosion resistive coatings for the protective shields of helicopter rotor blades.
There is a need for a method which can provide a fine grained, hard wearing ceramic surface that has low micro-roughness, low defect density, and the ability to withstand post-deposition heat treatment, if necessary, without detriment and degradation of the coating while securing high flexibility at a coating thicknesses exceeding 20 μm, and having high erosion and corrosion resistance properties as well as high reflectivity of the sun's radiation.
All patents, patent applications, provisional patent applications and publications referred to or cited herein, are incorporated by reference in their entirety to the extent they are not inconsistent with the teachings of the specification.